His heart began beating just a little faster Wednesday, when the Giants told rookie defensive lineman Cornelius Griffin that he had a special assignment for yesterday’s NFC Divisional playoff game against the Eagles.

By the time he stepped out of the shower yesterday, Griffin was ready to do the finest espionage job of his career.

“I was really excited,” said Griffin. “I knew I had to cool my jets.”

Griffin did just that, playing under control, and in the process, he cooled off McNabb, who had 198 yards in total offense, 51 less than he averaged this season. McNabb, who averaged 39 yards rushing per game, got just 17 against Spy Man and Co.

“I thought there was more than one guy spying the way they played,” said a dejected McNabb.

Giant defensive coordinator John Fox said the team had thought about using Griffin in the spy role earlier this season but didn’t want to heap too much responsibility on the second-round draft pick from Alabama. But Griffin isn’t a rookie after 16 games and at 6-foot-3, 300 pounds, he’s one of the few players who has the size and speed to spy McNabb.

It was such a plumb assignment that almost every player on the Giants wanted a shot. Fox knew Griffin and Barrow were ideally suited for this undercover job.

“Keith Hamilton really wanted to do it,” said Barrow. “But Fox has this smooth way of doing things. He kept telling Keith, ‘You’re too good on the line. We can’t take you off the line.’ Besides, Griffin’s been saying he’s the fastest 300 pounds in football. He got a chance to prove it.”

Griffin, who finished with four tackles and one and one-half sacks, said he watched more film than usual this week, not because of his assignment but because this was his first playoff game. He didn’t tell friends or even family members that they should watch yesterday’s playoff game because everywhere No. 5 went, No. 97 would be in the picture.

Griffin said McNabb got tired of seeing him come the fourth quarter, when the Giants had built a 20-3 lead. There was no trash talking, just an acknowledgment of what was taking place on the field.

“A couple of times he looked at me like, ‘Man, not you again,’ ” said Griffin. “I looked back like, ‘Yeah, all night.’ “